Ultra-high molecular weight, water soluble synthetic and natural polymers are used in a variety of important industrial and other applications. Better knowledge of the molecular weight distribution of these polymers could contribute significantly to their design and also provide a better understanding of their behavior in given applications. There have been numerous attempts to characterize the molecular weight distribution of these polymers by conventional size exclusion chromatography (SEC). The principal shortcomings associated with the application of this analytical technique to these polymers are poor resolution of ultra-high molecular weight polymers and poor detector response. Column packing materials generally have an exclusion limit of .about.2M. That is, SEC can generally be used to size molecules &gt;2M. The molecular weight distribution of important water soluble polymers can range up to .gtoreq.30M based on sedimentation experiments. Application of SEC methods provides little if any meaningful information about these ultra-high molecular weight materials with molecular weights &gt;2M.
Due to the limitations of SEC methods, other techniques have been investigated for characterizing ultra-high molecular weight polymers. Holzwarth (Carb. Res., 66, 173-186, 1978) described ultracentrifugation experiments using fluorescence detection to characterize xanthan polysaccharide polymers. These experiments require the use of highly complex apparatus which is not generally suitable for routine analysis work. In addition, despite the report of encouraging results, the technique has not found wide acceptance.
Theoretically, light scattering experiments would be extendable to these characterizations of molecular weight. However, in practice, this method has experimental constraints which can produce unreliable data due to alteration of the sample by necessary sample preparation. In addition, no distribution information is available from this method of analysis.
Yet another approach to developing methods for characterizing these polymers has been the method of hydrodynamic chromatography first developed by Small for sizing colloidal particles (U.S. Pat. No. 3,865,717) and later extended in a modified fashion to the field of water soluble polymers, such as dissolved sodium polystyrene sulfonate fractions (U.S. Pat. No. 4,280,923). In contrast to conventional SEC methods where separation occurs by diffusion into the pores of a porous packing material, HDC separations occur and are due to flow through the interstitial spaces of a porous bed of nonporous packing material. Small's attempt to use this method to characterize water soluble polymers by molecular weight was only partially successful. That is, in none of the work described by Small is the method extended beyond the conventional limits of SEC. Hence, both HDC and SEC as heretofore developed have not been successfully used to characterize high molecular weight polymers of greater than 2 million molecular weight.